Page 537 - Instrumentation Reference Book 3E

520 Nuclear instrumentation technology

more detail later, but Cherenkov detectors and of such particles or radiation through supersa-
cloud chambers are specialized research tools and turated gas.
are not discussed in this book. Photographic film can be used to detect the
passage of ionizing particles or radiation, since
they produce latent images along their paths in
22.1.21 Gus detecloer
the sensitive emulsion. On development, the
These include ionization chambers, gas propor- grains of silver appear along the tracks of the
tional counters. Geiger counters, multiwire pro- particles or ions.
portional chambers. spark counters, drift counters,
and cloud chambers.
~et~ct~~s
22.1.2.6 P ~ a ~ f ~ c ~ ~ ~ ~
c(~u~i~e~s
22.1.2.2 ~c~nti~~~~~o~ Thin (5 pm) plastic films o~pol~rcarbonate can be
used as detectors of highly ionizing particles
Some substances have the property that, when which can cause radia~ion damage to the niol-
bon~~arded with nuclear particles or ionizing ecules of the polycarbonate film. These tracks may
radiation, they emit light which can be picked be enlarged by etching with a suitable chemical and
up by a suitable highly sensitive light detector visually measured with a microscope.
which converts the light pulse into an electronic Alternatively, sparks can be generated between
pulse that can be amplified and measured. two electrodes, one of which is an aluminized
mylar film, placed on either side of a thin, etched
22.1.2.3 Cherenkov defectors polycarbonate detector. The sparks that pass
through the holes in the etched detector can be
When a charged particle traverses a transparent counted using a suitable electronic sealer.
medium at a speed greater than that of light
within the medium, then Cherenkov light is pro-
duced. Only if the relative velocity 8 = vIc and 22.1.2.7 Thermoluminescenr detectors
the refractive index n of the medium are such that
n/9 1 will the radiation exist. When the con- For many years it was known that if one
dition is fulfilled, the Cherenkov light is emitted heated some substances, particularly fluorites
at the angle given by the relation and ceramics, they could be made to emit light
photons and, in the case of ceramics, could be
(22.11) made incandescent. When ionizing radiation is
absorbed in matter most of the absorbed
energy goes into heat while a small fraction is
where I3 is the angle between the velocity vector used to break chemical bonds. In some mater-
for the particle and the propagation vector for ials a very minute fraction of the energy is
any portion of the conical radiation wavefront.
stored in metastable energy states. Some of
the energy thus stored can be recovered later
22.1.2.4 Solid-state detectors as visible light photons if the material is heated,
Some semiconductor materials have the property and this phenomenon is known as thermo-
that when a potential is applied across them, and luminescence {TL).
an ionizing particle or ionizing radiation passes In 1950 Darnels proposed that this phenom-
through the volume of material, ions are pro- enon could be used as a ineas~ement of radi-
duced just as in the case of a gas-ionization cham- ation dose, and in fact it was used to measure
ber, producing electronic pulses in the external radiation after an atom-bomb test. Since then
conrieetions which can be amplified, measured, interest in TL as a radiation dosimet~r has
progressed to the stage that it could now well
or counted. A device can thus be made, acting
film
like a solid ionization chamber. The materials replace pho~o~raph~c as the approved per-
sonnel radia~ioii badge carried by people who
that have found greatest use in this ap~lication may be involved with rad~oact~ve materials or
are silicon and germanium.
radiation.
22.1.2.5 Cfod c~~~i?~~e~s
22.1.2.8 ~ ~ for TI, adosimetry e ~ ~ ~ ~ ~
~
These were used in early research work. and are
still found in more sophisticated forms in high- The most popular phosphor for dosimetric pur-
energy research laboratories to demonstrate poses is lithium fluoride (LiF). This can be nat-
visnally (or photographically) the actual paths ural LiF, or with the lithium isotopes 6Li and 'Li
of ionizing particles or radiation by means of enriched or depleted, as well as variations in
trails of liquid droplets formed after the passage which an added activator such as manganese